The invention relates to a method and apparatus for determining the moisture content of a material by way of measuring the dielectric constant of the material, wherein a pulsed signal is applied to a proximate end of a measuring line extending through the material and is reflected at the distal end of the measuring line and the reflected signal is detected at the proximate end and compared with a threshold value and also the time delay between the signal application and the return of the reflected signal is evaluated.
Such a method and apparatus is disclosed for example in EP 0 478 815, which is assigned to the assignee of the present application and is marketed by the Assignee with considerable success. In this known method, a square signal is applied to a measuring cable. The pulse direction of the signal is greater than twice the travel time of the signal in the measuring cable. At the input of the measuring cable or, respectively, the output of the measuring signal generator the respective amplitudes are therefore superimposed thereby generating a summing signal formed by the input signal and the signal reflected from the distal end of the cable.
Although this known method provides very good results, it does have the disadvantage that the amplitude of the reflected signal affects the measuring accuracy. This is disturbing since the amplitude of the reflected signal depends on the electric conductivity and, consequently, the moisture content of the material. Furthermore, the ambient temperature around the measuring cable affects the amplitude of the reflected signal.
U.S. Pat. No. 6,477,474 B2 discloses a method for determining the dielectric constant of a material wherein, by means of an impulse generator, a pulse signal is supplied to a measuring cable and the travel time up to the reception of the reflected signal is determined and the dielectric constant is determined therefrom. A threshold value control circuit is provided by which a threshold value can be adjusted to a fixed threshold value or it can be adjusted depending on the reflected signal. However, definite criteria for the adjustment of the threshold value are not available from this publication.
Furthermore, U.S. Pat. No. 5,610,611 discloses an impulse reflectometer wherein an automatic threshold value adjustment occurs depending on the size of the reflected signal.
It is the object of the present invention to provide a method and apparatus for determining the moisture content of a material by which the accuracy of the measurement is improved.